You are taking a test where the final score is the number right minus the number wrong and the higher the score the better. If you think the answer is choice (a) with a certainty of x%, what should you do. That is relative to the value of x, should you not answer the question (0 points) or give (a) as the answer to the question (you expect y point on average where y is non-negative).
HINT: Taking the test 'a number of times' you would expect to get the answer right N times depending on x and the answer wrong M times depending on the value of x, so the average number of points you would get is
(N-M)/(N+M).
SOURCE: Me, the devilishly cleaver something or another.